1. Field of the Invention
The present invention relates generally to apparatus for use in drilling directional boreholes. More specifically, the present invention is related to stabilizer assemblies carried by a drillstring for altering the direction of drilling from vertical.
2. Background Information
The earliest efforts to drill directionally for petroleum hydrocarbons employed mechanical whipstocks, which were used to deflect a rotating drillstring from vertical in a previously vertical wellbore. The chief drawback to the use of whipstocks is that directional control of the bit and drillstring is lost once the drillstring is kicked off or deflected by the whipstock. Additionally, whipstock operations are time-consuming and therefore expensive.
Another method of directional drilling uses a bent or bendable sub with a downhole motor or turbine. The bent sub has a bend formed therein to position the drill bit a few degrees from the vertical axis of the remainder of the drillstring. A downhole motor is coupled between the bent sub and drill bit or is incorporated in the bent sub itself. The drillstring and downhole motor may be rotated to cause the bit to disintegrate formation and drill straight ahead at the same angle and azimuth of the existing borehole. When altering the direction of drilling is desirable, rotation of the drillstring is stopped and the bit is rotated by the drilling motor. This mode of operation is known as the "sliding" mode, because the drillstring is sliding rather than rotating with respect to the sidewall of the borehole. In the deviated portion of the borehole, the drillstring experiences sufficient frictional contact with the sidewall of the borehole to make it difficult to apply significant weight to the bit, resulting in reduced rates of penetration compared with rotary drilling. Examples of bent sub or motor directional drilling systems and method are disclosed in U.S. Pat. No. 5,311,953, May 17, 1994 to Walker; U.S. Pat. No. 5,139,094, Aug. 18, 1992 to Prevedel et al; and U.S. Pat. No. 5,050,692, Sep. 24, 1991 to Beimgraben.
In another directional drilling system and method, a pair of stabilizers is provided in the drillstring and are spaced-apart above the drill bit. The difference in diameter between the upper stabilizer and the near-bit stabilizer, whether adjustable or fixed, and the spacing between the stabilizers, provide lateral forces that assist in deflecting the bit from the vertical axis of the borehole. Such stabilizer arrangements are employed in both rotary drilling and downhole motor arrangements. If the stabilizers are adjustable and employed in surface rotation drilling, each stabilizer blade must extend from the stabilizer body the same distance to maintain symmetry and avoid eccentricity and associated rough running. If drilling is accomplished with a drilling motor, no such limitation is imposed on the upper stabilizer, above the drilling motor, because it is not rotated. Examples of stabilizer arrangements are found in U.S. Pat. No. 5,332,048, Jul. 26, 1994 to Underwood et al; U.S. Pat. No. 5,293,945, Mar. 15, 1994, to Rosenhauch et al.; U.S. Pat. No. 5,181,576, Jan. 26, 1993 to Askew et al.; and U.S. Pat. No. 4,754,821, Jul. 1, 1988 to Swietlik.
A variation on the adjustable stabilizer theme is to provide stabilizer bodies having fixed stabilizer blades, but having pistons acting between the drillstring or stabilizer sub and the fixed stabilizer bodies to introduce eccentricities between the upper and lower stabilizers and resulting lateral deflection forces. These arrangements require multiple piston actuations per revolution of the drillstring and thus present mechanical and reliability disadvantages. Examples of such arrangements can be found in U.S. Pat. No. 5,038,872, Aug. 13, 1991 to Shirley and U.S. Pat. No. 3,593,810, Jul. 20, 1971 to Fields.
U.S. Pat. No. 4,947,944, Aug. 14, 1990 to Coltman et al. discloses a stabilizer that employs electric motors to actuate stabilizer blades independently in a stabilizer sub that rotates independently of the drillstring to which it is coupled. This permits the stabilizer blades to remain stationary relative to the borehole and simplifies the process significantly. One drawback to the Coltman device is that is does not appear to be collapsible in a "fail-safe" state to a reduced radial dimension in the event the stabilizer becomes stuck in the borehole due to malfunction.
A needs exists, therefore, for a directional drilling assembly or system for use with an efficient rotating drillstring that permits the driller to control precisely the trajectory of the bit during drilling operation and that is capable of being withdrawn from the borehole relatively easily in the event of malfunction.